1. Field of the Invention
The present invention relates generally to a phase detector for detecting the phase of an input signal, and more particularly to a phase detector used in a demodulator of a receiving apparatus such as a radio receiver, a TV receiver, a portable telephone or a cordless telephone.
2. Description of the Related Art
A direct conversion (DC) system is known as one of detection and demodulation systems for use in a receiving apparatus for receiving a signal having phase information. In the direct conversion (DC) system, a received signal is subjected to quadrature demodulation by using two reference signals having frequencies which are equal to a carrier frequency and different from each other by 90.degree.. Thus, the received signal is directly converted to a base-band range signal. Unlike a heterodyne system, there is no need to use an intermediate frequency stage, and the receiving apparatus can be reduced in size and cost.
On the other hand, the direct conversion system needs to perform automatic gain control (AGC) to keep the level of a received signal constant, in the case where the dynamic range of the received signal is wide, e.g. 90 dB or more, as in an environment where a receiving field strength varies greatly, e.g. an environment of mobile communication. However, AGC, which is a feedback loop, cannot follow a rapid variation of the intensity in received electric field.
Unlike the heterodyne system, in the direct conversion system, AGC cannot be performed at the intermediate frequency stage and some means needs to be provided from an aspect of the system. Specifically, in the heterodyne system, a limiter amplifier is provided in the intermediate frequency stage to limit amplitude and necessary phase information alone is taken out. Accordingly, even if the dynamic range of the received signal is wide, demodulation can be performed exactly. Furthermore, in the heterodyne system, if a received signal is a QPSK modulated signal, the received signal is passed through a proper wave shaping filter for reducing inter-code interference and then subjected to amplitude limitation. Thereby, good demodulation characteristics can be achieved. In the direct conversion system, however, no stage is provided for converting a received signal to a signal with a constant frequency, e.g. an intermediate frequency signal. Consequently, it is not possible to keep the level of the signal constant by means of a limiter amplifier.
In order to cope with this problem, it has been proposed (in Jap. Pat. Appln. KOKAI No. 58-196629) that in the direct conversion system, a received signal is converted directly to a base band signal by quadrature demodulation and the resultant signal is further frequency-converted to an intermediate frequency signal, and then the intermediate frequency signal is subjected to amplitude limitation. This system, however, requires a frequency reconverting circuit and a limiter circuit, resulting in a very complex structure. Consequently, the advantages of the direct conversion system, i.e. small and inexpensive structures are lost.
As has been described above, in the conventional receiving apparatus of the direct conversion system, in order to process a received signal with a wide dynamic range, a signal is directly converted to a base band signal and the converted signal is subjected to frequency reconversion and then to amplitude limitation. With this prior art, the structure is very complex and an inexpensive receiving apparatus having a small size and high performance is difficult to obtain.